This invention relates to nuclear magnetic resonance (NMR) methods and apparatus. More specifically, this invention relates to magnet shimming techniques based on magnetic field measurements deduced from chemical shift imaging (CSI) of a homogeneous volume.
The nuclear magnetic resonance phenomenon has been utilized in the past in high resolution NMR spectroscopy instruments by structural chemists to analyze the structure of chemical compositions. More recently, NMR has been developed as a medical diagnostic modality having applications in imaging the anatomy, as well as performing in vivo, non-invasive, spectroscopic analyses. As is now well known, the NMR phenomenon can be excited within a sample object, such as a human patient, positioned in a polarizing magnetic field B.sub.o by application, in a well-known manner, of pulse sequences comprised of RF and gradient magnetic field pulses. The NMR signals, induced by the excitation, emanating from the object are then detected and used to derive NMR imaging or spectroscopic information about the object studied.
The advent of whole-body NMR imaging and spectroscopic techniques has led to stringent requirements for homogeneity of the polarizing magnetic field over large volumes in wide-bore magnets. The polarizing magnetic field may be provided by a number of magnet types, such as permanent magnets, resistive electromagnets, or superconductive magnets. The latter are particularly desirable in that high fields may be created and maintained without large energy input requirements. However, in all of these NMR magnet types, the intrinsic magnetic fields are generally not sufficiently homogeneous to perform the desired imaging or spectroscopic measurements to desirable resolutions for medical application. Most standard NMR imaging and spectroscopic pulse techniques require a field homogeneity better than about .+-.4 ppm. (.+-.250 Hz. at 1.5 Tesla) over the volume of interest. Some chemical shift imaging (CSI) techniques require much better homogeneity (less than 1 ppm). In-vivo spectroscopy of carbon (.sup.13 C) and phosphorus (.sup.31 P) places even more severe requirements on the measurement and correction of heterogeneity especially with the presence of field distortion caused by the object studied, itself.
To improve the homogeneity of the polarizing magnetic field, produced by the main magnet, magnet systems typically include correction coils having a variety of geometries. The correction coils, which are also known as shim coils, include both axisymmetric coils and transverse coils. The axisymmetric coils are generally disposed in a helical pattern on a cylindrical coil form while the transverse correction coils are generally disposed in a so-called saddle shape. Each coil has its own current control and ideally is designed to produce a field orthogonal to that of all the other coils. In practice, some coils, such as Z.sup.2 X and Z.sup.2 Y, produce fields which have nonorthogonal components. Correction of field heterogeneity involves adjustment of the individual shim coil currents so that the combined fields of the shim coils just balance the error field in the polarizing magnetic field produced by the main magnet. The procedure is often referred to as shimming.
The need exists for a quick and accurate (and, ideally, automated) method for shimming the main magnet. Several approaches have been used in the past. In one class of techniques, measurements of field intensity at strategic locations within the magnet bore are utilized to optimize a given shim current energizing a particular shim coil. This technique is often employed in the initial stages in magnet setup to approximate the desired degree of field homogeneity. As an example, if magnetic field measurements are obtained with a magnetometer probe along a given axis, the shim currents in the shim coil designed to correct the field along that axis can be adjusted. A difficulty which arises with this and other prior art methods to be described is that field measurements are made in air so that field distortions caused by the object studied are not compensated.
In a second class of techniques, the linewidth (or equivalently, the decay time) of a free-induction decay (FID) signal derived from a suitable specimen is monitored while the shim coil currents are adjusted either by hand or by computer in an iterative fashion. One such technique known as Simplex is described by W. Spendley, et al in an article entitled "Simplex Algorithm," in Techrometrics, Vol. 4, p. 441 (1962). In using such techniques, care must be taken to avoid local minima during such "steepest descent" methods. Often, symmetry considerations can be employed, as described by F. Romeo, et al in "Magnet Field Profiling: Analysis And Correcting Coil Design," in Magnetic Resonance in Medicine, Vol. 1, p. 44 (1984), in the choice of phantom shape, and its deployment to diminish the importance of the selected shim components. Such iterative techniques are tedious at best, unless the approximation to the desired magnet homogeneity is already rather good, and do not satisfy the requirement for rapidity.
A third approach to shimming has evolved in accordance with which the polarizing magnetic field is measured at many points within or enclosing a volume of interest and expanded in an orthonormal basis set to calculate the contribution required from each coil component to achieve a homogeneous magnetic field. Thus, if the measurements are accurate and the expansion contains a sufficient basis set, the estimate of required changes to shim currents can be arbitrarily precise. A drawback associated with this technique is that a magnetometer probe mounted on a carriage is often used to sample the magnetic field at various points within the volume of interest. If sufficient measurements are not made, because, for example, of the relative tediousness with which measurements are made with the probe, this technique requires the application of iteration in order to reach a satisfactory level of homogeneity. This technique will be described in greater detail hereinafter.
It is, therefore, a principal object of the invention to provide a method for quickly and accurately shimming the magnet of an NMR system without the attendant drawbacks of the techniques described above.
It is another object of the invention to provide an automated data acquisition technique which more uniformly covers the volume of interest.
It is still another object of the invention to provide a data acquisition technique capable of providing magnetic field measurement and compensation in the presence of the object (e.g., patient) being studied.